In each of Patent Literatures 1 and 2 listed below, there is disclosed a reference light source enabling calibration of a spectral radiance meter. The reference light source disclosed in each of those literatures includes an integrating sphere, which, a light ray from a light emitting diode (LED) or a semiconductor laser enters from outside, and the light ray multiple-reflected inside the integrating sphere exits from a radiance reference plane, which is an opening formed in an outer wall of the integrating sphere. The spectral radiance meter to be calibrated is placed to be opposed to the radiance reference plane to measure a radiance of the radiance reference plane. Based on the thus measured radiance and a probable radiance of the above-mentioned radiance reference plane, which is measured separately by other measurement means, the spectral radiance meter to be calibrated is calibrated.